High molecular weight polyesters and particularly polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
For certain applications, such as injection or extrusion, and blow molding or blown film extrusions, it is desirable to use polyester resins that have lower melt viscosity. In addition, certain polyesters are known to interact with selected additives, for instance, aromatic carbonates and (poly-) carbonates during compounding to give very high molecular weight/melt viscosity products Such carbonate species may include, e.g., decabromodiphenyl carbonate, copolycar bonates of bisphenol-A and tetrabromobisphenol-A, bisphenol-A polycarbonate, tetrabromobisphenol polycarbonate, and the like. Increase in melt viscosity is particularly undesirable when the polyester is compounded with bisphenol-A (BPA)-tetrabromo BPA copolycarbonate (flame retardant), and Sb.sub.2 O.sub.3 (synergistic flame retardant additive) that, unavoidably, also act as a catalyst in the linear polyesterpolycarbonate reaction. Prior art methods are known to prevent the build up in melt viscosity in such high molecular weight polyesters. One such method is to add carboxylic acids, such as isophthalic acid, to the polyesters. Although this technique is very effective in reducing the melt viscosity of the resulting composition, the carboxylic acids, and particularly isophthalic acid, being volatile, tend to "plate out" on the mold; that is, they tend to, after continuous molding operations, form an undesirable residue on the mold surface.